Thallium: the essentials

When freshly exposed to air, thallium exhibits a metallic lustre, but soon develops a bluish-grey tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife.

The element and its compounds are toxic and should be handled carefully. Thallium may cause cancer.

Thallium: historical information

Thallium was discovered by Sir William Crookes in 1861 at England. Origin of name: from the Greek word "thallos" meaning "green twig" or green shoot.

Thallium was discovered spectroscopically in 1861 by Crookes. The element was named after the green spectral line, which identified the element (Greek "thallos", green twig). The metal was isolated both by Crookes and Lamy in 1862. They had been expecting to isolate tellurium after removing selenium from the byproducts from a commercial sulphuric acid factory but instead foundthe new element thallium.

Thallium has no biological role. Thallium compounds are extremely toxic. Their effects are cumulative and they can be absorbed though the skin. Thallium poisoning takes several days to act and it affects the nervous system.

Thallium minerals are rare but include crooksite, lorandite, pyrites, and hutchinsonite. Thallium is also associated with potassium minerals such as sylvite and mineral pollucite. Thallium is also produced as a byproduct from zinc and lead refining plants, as well as from particular sulphuric acid factories.

Isolation

Isolation: thallium metal would not normally be made in the laboratory as it is available commercially. Crude thallium is present as a component in flue dust along with arsenic, cadmium, indium, germanium, lead, nickel, selenium, tellurium, and zinc. This is done by dissolving in dilute acid, precipitating out lead sulphate, and then adding HCl to precipitate thallium chloride, TlCl. Further purification can be achieve by electrolysis of soluble thallium salts.

Thallium has two stable isotopes and one of these, Tl-203, is used to produce one of the (workhorses( of nuclear medicine: Tl-201. Tl-201 is used extensively for imaging and in particular for perfusion tests of the myocardium. These tests are done to determine the damage to the heart from a heart attack or from heart diseases. Tl205 has been proposed as an alternative target for the production of Tl-201. Tl-205 is also used in nuclear magnetic resonance research.